1. Field of the Invention
The present invention relates to telecommunication techniques and has been developed with particular attention paid to its possible application to telecommunication systems based upon the standard CDMA/3GPP TDD (acronyms for Code Division Multiple Access/Third Generation Partnership Project and Time Division Duplex) 3.84 Mcps option.
2. Description of the Known Art
In order to enable acquisition of a base station by a mobile terminal included in a telecommunication system based upon the standard 3GPP TDD mode or the like, the corresponding receiver needs means capable of performing the function of frame synchronization and identification of the so-called codegroup. The possibility of executing these functions is essential for performing the subsequent steps in the framework of the cell-search system.
In particular, when a mobile terminal is turned on, it does not have any knowledge of the timing of the transmitting cell on which it is to be assigned. The 3 GPP standard then proposes a procedure of initial cell search to acquire the signal of the cell and synchronize therewith.
In the case in question, this procedure consists basically of 3 steps:                1. acquisition of the synchronization slot (this is a “coarse” slot synchronization, where by the term “coarse” is understood the fact that the presence and the rough position of the synchronization channel, or SCH, is acquired, but it is not yet possible to define the initial instant of the slot of which the SCH forms part; in this connection, see FIG. 1 of the annexed drawings [SCH-slot_position]);        2. (fine) slot synchronization, identification of parity of the frame number and identification of the cell codegroup; and        3. identification of the scrambling code (third step) and of the cell parameter.In the embodiment of the second step described above, it is assumed that the acquisition of the synchronization slot (or SCH slot) has previously been obtained with a first coarse slot synchronization in the course of the first step.        
At this point, to obtain fine slot synchronization, define frame parity (i.e., whether the frame number is even or odd), and identify the cell codegroup, to which there is associated the cell offset, in the second step there is used the secondary synchronization channel (SSCH), on which there is transmitted, within each synchronization slot, a set of three codes or words of 256 chips each.
The 3.84 Mcps version of the TDD standard uses a subset of 12 of the 16 secondary synchronization codes already in use for the FDD (Frequency Division Duplex) version.
The sixteen 256-chip complex codes used in the standard are generated on the basis of the following rules:                a first sequence b at a chip-rate with a repetition period equal to 16 (i.e., a repetition every 16 elements) is multiplied by a sequence 16 times slower according to the two formulae given below, to obtain the base sequence z:z=<b,b,b,−b,b,b,−b,−b,b,−b,b,−b,−b,−b,−b,−b,>b=<1,1,1,1,1,1,−1,−1,−1,1,−1,1,−1,1,1,−1,>        
The base sequence z is then multiplied, element by element, with a Hadamard code of length 256, which is chosen on the basis of the following rule: defining by m the number identifying the secondary synchronization code (SSC) to be generated, the Hadamard code number by which to multiply the sequence z is equal to 16×(m−1), with m ranging from 1 to 16.
In the solutions known to the art, for example, from the international patent application WO-A-00/74276, used as model for the preambles of Claims 1 and 6, the performance of the second step of the cell search envisages that the secondary synchronization codes SSC, contained in the secondary synchronization channel (SSCH), are extracted by means of a correlation process. The samples of the received signal are correlated with the possible secondary synchronization codes SSC transmitted on the channel SCCH. There is then identified the set of three codes that has the highest correlation energy, and there are then used the steps associated to the codes of said set of three in order to define, according to the standard, the codegroup parameters and other parameters for frame synchronization, such as slot offset and frame number (even or odd frame).
This solution is schematically represented in the diagram of FIG. 2, where the reference number 10 designates a bank of twelve complex finite-impulse-response (FIR) filters, which are coupled to the twelve possible secondary synchronization codes SSC. The samples of the received signal (r) are sent at input to the bank 10 of FIR filters, and at the twelve outputs of the bank 10 there are generated signals indicating the correlation energies corresponding to said codes SSC. These signals are sent to a system for detecting the maximum value designated by 11.
The system for detection of the maximum value 11 identifies a given number (equal to three) of codes SSC provided with highest correlation energy, which are sent to a comparison block designated by 12.
Block 12 executes an operation of comparison with a table that gives—according to the possible combinations of the phases of the set of three codes SSC identified—corresponding codegroups CD, slot offset OS, and frame number FN, which are then supplied at output from said comparison block 12.
The solution according to the known art, represented in FIG. 2, thus requires a huge number of FIR filters, one for each code SSC of which it is necessary to obtain the correlation energy. This entails a considerable expenditure in terms of memory cells. In fact, a correlator based upon a FIR filter requires 256×2 memory cells, it being necessary to operate on 256-chip codes SSC. Furthermore, in order to store the signals indicating the correlation energies, further memory cells are necessary. The utilization of a very high number of memory cells implies the use of a considerable area on the chip designed for identification of the codegroup, as well as a considerable power consumption.
Even though in what follows, for reasons of clarity and simplicity of exposition, practically exclusive reference will be made to this application, it is in any case to be borne in mind that the scope of the invention is more general. The invention is in fact applicable to all telecommunication systems in which there arise conditions of operation of the type of the ones described in what follows. By way of non-exhaustive example, reference may be made to satellite telecommunication systems and mobile cellular systems corresponding to the standards UMTS, CDMA2000, IS95 or WBCDMA.